1. Field of the Invention
The invention concerns an integral servo steering gear with a rack-type transmission wherein the control mechanism is disposed in the working piston.
2. Description of the Prior Art
Servo steering gear is known wherein a portion of the working piston is formed as a rack meshing with the teeth of a toothed sector gear of a steering shaft. In this way the advance of the working piston is translated into rotary motion of the steering shaft by the rack-and-pinion connection. Servo steering gear provided with rack and sector gear transmission frequently has the control mechanism disposed in the working piston see example U.S. Pat. No. 3,722,368. This constructional arrangement is such that a control sleeve is closely fitted to the control piston and is secured against axial and angular displacement so as to form a single constructional unit with the piston. As a consequence of the tight fitting, the hydraulic fluid - usually oil - channels leading to the control mechanism are for the greater part arranged in the piston. On the other hand, the control piston is disposed in the control sleeve so as to be axially or angularly displaceable corresponding to the magnitude of the controlling displacement and is connected with a steering nut. The clearance between the control piston and the control sleeve is of the order of hundredths of millimeters.
It is a disadvantage of this arrangement that, due to the great load on the rack and sector gear connection, the piston is subjected to resilient deformation, as a result of which the annular clearance between the piston and the control sleeve is deformed and the control piston may stick. This in turn has the consequence that the proportionality between the manual pressure and the servo pressure is destroyed, thus the servo-steering becomes insensitive, as it were, and the operation becomes faulty.
Another disadvantage of the known arrangements is that the rack and sector gear connection is not central, i.e. does not fall on the axis of the piston, as a result of which the working piston rocks very slightly in the steering casing or cylinder. This rocking is allowed to happen because of the clearance between the piston and the bore of the steering casing in which it is disposed and because of the resilient deformation. As a consequence of the rocking of the piston, the steering nut journalled by bearings in the piston is also rocked relative to the steering spindle and this causes a certain stress to arise in the threaded connection between the steering nut and the steering spindle as well as in the bearing arrangement of the steering nut and the steering spindle. Since there is a connection between the steering nut and the control piston, the stress in the steering nut has an effect on the control piston also.
Practical drawbacks of the known arrangement are that the bore of the control sleeve can only be finish-machined after the sleeve has been secured in the piston and that the control mechanism may only be examined together with the piston.